Honestly I think it's just budget. Effects team can add in glowy shit around fists without any problems.
Stretchy arms and giant fists are hard, first you have to do a lot of work to make it not look incredibly stupid and cheap. Then you have to do special filming t make it work with actors, like having a small girl swing a car sized fist at a guy and then film some placeholder for the car sized fist, and then get reactions etc from the guy, and the background and all that shit. The alternative is she stretches maybe once per episode, if at all.
It's ten times more work and all they want is the minority points and action figure sales.